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+<title>Chapter 6. OOP</title>
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+<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch6oop">
+<div class="titlepage"></div>
+<div class="toc">
+<p><b>Table of Contents</b></p>
+<dl><dt><span class="article"><a href="#id2582702">Classes and Objects</a></span></dt></dl>
+</div>
+<div class="article" title="Classes and Objects">
+<div class="titlepage">
+<div><div><h2 class="title">
+<a name="id2582702"></a>Classes and Objects</h2></div></div>
+<hr>
+</div>
+<p id="ch6oop_1"></a>In the previous sections we learnt about functions which provide certain level
+of abstraction to our code by holding the code which performs one or more
+specific functionalities. We were able to use this function as many times as we
+wanted. In addition to functions, Python also higher level of abstractions
+through <span class="emphasis"><em>Classes</em></span> and <span class="emphasis"><em>Objects</em></span>. <span class="emphasis"><em>Objects</em></span> can be loosely defined as a
+collection of a set of data items and a set of methods. The data items can be
+any valid Python variable or any Python object. Functions enclosed within a class
+are called as <span class="emphasis"><em>methods</em></span>. If you are thinking if methods are functions why is there
+a distinction between the two? The answer to this will be given as we walk through
+the concepts of <span class="emphasis"><em>Classes</em></span> and <span class="emphasis"><em>Objects</em></span>. <span class="emphasis"><em>Classes</em></span> contain the definition for the
+<span class="emphasis"><em>Objects</em></span>. <span class="emphasis"><em>Objects</em></span> are instances of <span class="emphasis"><em>Classes</em></span>.</p>
+<p id="ch6oop_2"></a>A class is defined using the keyword <span class="strong"><strong>class</strong></span> followed by the class name, in
+turn followed by a semicolon. The statements that a <span class="emphasis"><em>Class</em></span> encloses are written
+in a new block, i.e on the next indentation level:</p>
+<pre class="programlisting"> class Employee:
+ def setName(self, name):
+ self.name = name
+
+ def getName(self):
+ return self.name</pre>
+<p id="ch6oop_3"></a>In the above example, we defined a class with the name Employee. We also defined
+two methods, setName and getName for this class. It is important to note the
+differences between the normal Python functions and class methods defined above.
+Each method of the class must take the same instance of the class(object) from
+which it was called as the first argument. It is conventionally given the name,
+<span class="emphasis"><em>self</em></span>. Note that <span class="emphasis"><em>self</em></span> is only a convention. You can use any other name, but
+the first argument to the method will always be the same object of the class
+from which the method was called. The data memebers that belong to the class are
+called as <span class="emphasis"><em>class attributes</em></span>. <span class="emphasis"><em>Class attributes</em></span> are preceded by the object of
+the class and a dot. In the above example, <span class="emphasis"><em>name</em></span> is a class attribute since it
+is preceded by the <span class="emphasis"><em>self</em></span> object. <span class="emphasis"><em>Class attributes</em></span> can be accessed from
+anywhere within the class.</p>
+<p id="ch6oop_4"></a>We can create objects of a class outside the class definition by using the same
+syntax we use to call a function with no parameters. We can assign this object
+to a variable:</p>
+<pre class="programlisting"> emp = Employee()</pre>
+<p id="ch6oop_5"></a>In the above example, we create an object named <span class="emphasis"><em>emp</em></span> of the class <span class="emphasis"><em>Employee</em></span>.
+All the attributes and methods of the class can be accessed by the object of the
+class using the standard notation <span class="emphasis"><em>object.attribute</em></span> or <span class="emphasis"><em>object.method()</em></span>.
+Although the first parameter of a class method is the self object, it must not
+be passed as an argument when calling the method. The <span class="emphasis"><em>self</em></span> object is implicitly
+passed to the method by the Python interpreter. All other arguments passing rules
+like default arguments, keyword arguments, argument packing and unpacking follow
+the same rules as those for ordinary Python functions:</p>
+<pre class="programlisting"> >>> emp.setName('John')
+>>> name = emp.getName()
+>>> print name
+John
+>>> print emp.name
+John</pre>
+<p id="ch6oop_6"></a>If we at all try to access a class attribute before assigning a value to it, i.e
+before creating it, Python raises the same error as it would raise for the
+accessing undefined variable:</p>
+<pre class="programlisting"> >>> emp = Employee()
+>>> emp.name
+Traceback (most recent call last):
+ File "class.py", line 10, in <module>
+ print e.name
+AttributeError: Employee instance has no attribute 'name'</pre>
+</div>
+</div></body>
+</html>